It seems Valve are quite serious about getting back into making games, as Campo Santo the developer of Firewatch has joined them.
Writing on their official website in a post, the team from Campo Santo confirmed that the news is true. They said they found that people at Valve share the same values that they do, so it became an "obvious match". Expanding on that, they said this:
We had a series of long conversations with the people at Valve and everyone shared the satisfaction we take in working with people whose talents dwarf our own to make things we never thought possible. Both sides spoke about our values and how, when you get right down to it, we, as human beings, are hard-limited by the time we have left when it comes to making the things we care about and believe in. They asked us if we’d all be interested in coming up to Bellevue and doing that there and we said yes.
They confirmed that their next game, In the Valley of Gods, is still being made and so it's now a Valve game.
Personally, I think it's great that Valve are bringing in some obviously talented folk to make games. Valve have a lot of resources and contacts that can help for sure. It should also mean they have good Linux support, since Valve are still committed to Linux gaming.
Since it will now be a Valve game, it will be interesting to see if In the Valley of Gods will make it to GOG. I wouldn't expect it to now, but maybe Valve will surprise us there.